The present invention relates to a sensor comprising zirconia base oxygen ion conductive solid electrolyte cells for measuring oxygen concentration or A/F-ratio ("A/F-ratio" means "air/fuel-ratio") in an exhaust-gas atmosphere of a combustion equipment, typically an internal combustion engine. Particularly, it relates to the structure of the sensor of the system combining a pump cell of above kinds and a sensor cell, and more specifically, a pump cell element for the pump cell.
For the measurement of oxygen concentration in the exhaust gas, there are usually used oxygen sensors of the system combining a pump cell and a sensor cell, which are comprised of zirconia base oxygen ion conductive solid electrolyte cell. This system is divided into two kinds of types, a closed compartment type and a gap compartment type. The Japanese Patent Kokai Publication No. 56-130649 discloses a closed compartment type sensor which measures an oxygen concentration in exhaust gas containing excess oxygen at a theoretical air-fuel ratio (about 14.5; excess air rate .lambda.=1) or more. And a gap compartment type sensor is disclosed in U.S. Pat. No. 4,498,968.
In either type of the sensors, an electric current I.sub.P for the pump cell is automatically adjusted in a pump cell disposed in an electric current controlling and measuring circuit for the pump cell, so that the output of the sensor cell produced by passing a pump electric current I.sub.P in a direction of pumping up oxygen to the side of exhaust gas from a closed compartment (or a gap compartment) may always indicate a certain constant value no matter how the oxygen concentration in the exhaust gas may change. In this case, I.sub.P changes in proportion to the oxygen concentration, which can be used to measure the oxygen concentration.
The voltage produced between the two electrodes of the pump cell becomes large at Po.sub.2 =0, that is, at an oxygen concentration nearly corresponding to I.sub.P =0. When this voltage is too large, a solid electrolyte plate (oxide sintered body) is reduced on the cathode side, and inner resistance comes to be increased due to occurence of so-called blackening. Further, when a pump electric current as well as an electric voltage are large, the blackening easily occurs.
Japanese Patent Kokai Publication No. 59-208453 discloses that in a gap compartment type sensor, the A/F-ratio of the exhaust gas at the fuel rich side (at the side of .lambda.&lt;1) is measured in the same system as in the case of .lambda.&gt;1.
Japanese Patent Kokai Publication No. 59-208455 discloses a system which causes a pump electric current to flow in the reverse direction in a gap compartment sensor, that is, the system which causes an electric current to flow in the reverse direction of transferring oxygen from the exhaust gas side to the gap compartment side, in order to measure the A/F-ratio of the exhaust gas at the fuel rich side.